This invention relates to spacecraft attitude control systems that use sensors to provide attitude information to a flight controller that controls reaction wheels to adjust attitude.
Many spacecraft, including satellites, have attitude control systems that use sun sensors, earth sensors and/or inertial sensors to determine attitude and reaction wheel assemblies (RWA) in an arrangement shown in FIG. 1 to rotate the spacecraft to an desired attitude. An attitude control system (ACS) receives signals from the sensors to determine spacecraft attitude, computes the required attitude and commands a change of speed for the reaction wheels, causing the spacecraft to rotate around its axes to the correct or desired attitude. An RWA is called an assembly because it comprises a wheel mass, electric motor and controls as a single unit. The ACS is part (one function) of an onboard computer system. Electrical power for the sensors and reaction wheels is provided over a central spacecraft power bus. The individual power requirements for the sensors and reaction wheels can be dramatically different, frequently between 20 and 100 volts. The practical effect is that each sensor and reaction wheel must include its own internal secondary power supply, which converts the spacecraft primary voltage to usable voltage levels for each component. Furthermore, each sensor and wheel must have a appropriate input/output (I/O) or converter protocol to communicate with the ACS, for instance the 1553 standard that is shown in FIG. 1.
This architecture introduces several system obstacles: Providing a secondary power supply for each sensor adds considerably to weight, complexity and cost. Because each sensor provides data directly to the ACS, its I/O must be compatible with the ACS I/O, which is more expensive and complex.
A spacecraft attitude control system using RWAs can be improved by providing secondary power to the sensors from power supplies that are included in the RWA and by passing the sensor signals through the RWA where they can be converted into a suitable format for the ACS, reducing the complexity and cost of the sensors. Moreover, one or more RWAs can contain a microcomputer to control the RWAs using the sensor signals independently (as an alternative) of the ACS. The sensor signals can be applied to RWAs configured to be primary RWAs and secondary or slave RWAs. The slave RWAs provide the sensor signals to the ACS if the primary RWAs are inoperative. Backup electrical power for the system can be generated from the rotating reaction wheels if the main spacecraft bus power is interrupted.